This application seeks support to study membrane transport in blood cells and tissue using electron probe microanalysis (EPMA). The electron probe is capable of elemental localization and quantitation on both cellular and subcellular levels. Transport problems which we plan to study utilizing established EPMA preparative techniques are: (1) heterogeneities in red cell transport, (2) role of hemoglobin in regulation of red cell volume, (3) sequence of membrane maturation and hemoglobin production in erythropoesis, (4) distal tubular phosphate transport in rat.